This invention relates to water-dilutable etherified melamine-formaldehyde resins, their preparation and use for preparing building materials and insulation material, fleece, web or nonwovens production, paper and textile finishing and preparing foils and edgings.
The preparation of water-soluble etherified melamine resins is common knowledge. First melamine (optionally mixed with other amino resin formers such as urea, acetoguanamine, benzoguanamine and caprinoguanamine) is methylolated by addition of formaldehyde or of compounds which release formaldehyde under the reaction conditions, and condensed, at pH 7 to 10 and 40 to 110xc2x0 C. and then the etherification alcohol is added and reacted at pH 1 to 7 and 30 to 80xc2x0 C. The condensation and etherification conditions depend on the properties desired of the resin, for example viscosity and water dilutability. Preferred etherification alcohols include methanol, ethanol, n-propanol, isopropanol and the isomeric butanols (n-, sec-, tert- and isobutanol), especially methanol, n-butanol and isobutanol.
The preparation of anionically modified melamine resins, i.e., cocondensates of melamine, formaldehyde, salts of amidosulfuric acid (sulfamic acid, sulfamidic acid, amidosulfonic acid), sodium sulfite or sodium bisulfite (sodium hydrogensulfite) and the like, is likewise known.
For instance, EP-A 0 176 709 (DE-A 34 30 248) describes condensates prepared by reaction of melamine, formaldehyde (M/F ratio=1:2 to 1:3.2 mol/mol), a sulfamate (0.025 to 0.045 mol of sodium sulfamate per 1 mol of formaldehyde; corresponds to 0.05 to 0.14 mol of the sulfamate per 1 mol of melamine) and a nonreducing sugar (in an amount of at least 3% of the mass of the reaction mixture) in the presence of a (thio)amide or amidine and of a C1- to C3-alcohol at a pH of above 8. The resultant resins are used as coatings, impregnants and binders for fiber webs. The resin solutions are said to be notable for very good stability in storage in particular. Under the reported alkaline reaction conditionsxe2x80x94pH above 8xe2x80x94and the small amounts of alcohols addedxe2x80x94below 0.35 mol of methanol per 1 mol of melamine in each of the reported examplesxe2x80x94etherification is impossible. The water dilutability of the resins described is below 1:7 (measured as mass ratio of resin solution to demineralized (DM) water), unlimited miscibility with DM water is not achieved.
DE-A 22 59 680 claims a process for preparing melamine-urea-formaldehyde resins having a mass ratio of melamine to urea in the range from 1:9 to 9:1 which are modified with organic aminocarboxylic or aminosulfonic acids or sodium or ammonium sulfite or bisulfite and etherified with alcohols containing 1 to 3 carbon atoms. The resins are used as chlorinefast finishing agents. The amino groups of the melamine and of the urea have to be completely methylolated to this end; the condensation products should ideally no longer contain any free NH groups. The process is carried out in two stages; the first stage comprises a methylolation under alkaline catalysis and incorporation of the anionic component by cocondensation and the second stage comprises an etherification with an alcohol under acidic catalysis. The resins having such a high degree of methylolation result in products with elevated formaldehyde cleavage on drying.
U.S. Pat. No. 3,487,048 (GB 1232031) describes a one step methylolation and etherification of melamine in an acidic medium. To this end, 3 to 6 mol of formaldehyde are reacted in methanol with melamine in the presence of acids, preferably amidosulfonic acid, as a catalyst to form a methyl-etherified melamine resin directly in one step. The amidosulfonic acid added serves only as a proton donor; incorporation into the resin is not described or stipulated for the resin properties. The amount of amidosulfonic acid used in the examples is 1.3 to 1.4 mmol per 1 mol of melamine. The problems of water dilutability and storage stability are not discussed.
The cleavage of formaldehyde in the course of curing of (methanol-) etherified melamine resins can be reduced by minimizing the ratio F/M of the amounts of substance of formaldehyde and melamine in the batch. On the other hand, the lower limit of F/M is determined by the solution properties of melamine in the course of the methylolation. In the case of resins having an F/M of below 1.9 it is difficult to obtain stable resins which preserve their high water compatibility over the storage period. It is likewise possible to use non-etherified melamine resins having an F/M of below 2 which have been modified with sulfamates or sulfites or bisulfites (hydrogensulfites) to obtain unlimited water compatibility. But the stability of these resins having an undiminished high water compatibility over the storage period is limited to just a few weeks.
It is an object of the present invention to provide aqueous melamine resins combining good stability in storage and unlimited water miscibility with low formaldehyde cleavage from substrates impregnated or finished therewith.
Aqueous melamine resins are melamine resins having a mass fraction of water of not less than 5%. Melamine resins of this type are said to possess unlimited water miscibility when there is no miscibility gap in the system consisting of melamine resin and water at room temperature (20xc2x0 C.). In macroscopic terms, the existence of a miscibility gap can be identified by the occurrence of opacity on gradual thinning of the pure melamine resin with water under intensive agitation. Aqueous melamine resins are said to possess good stability upon storage if there is no phase separation (opacity) for at least 6 weeks from their preparation.
Cocondensation of compounds having anionic groups into (methyl-) etherified melamine-formaldehyde resins provides novel resins having distinctly improved stability in storage coupled with unlimited water miscibility.
xe2x80x9cCompounds having anionic groupsxe2x80x9d for the purposes of the invention are such compounds which, when melamine resins modified therewith are dissolved in water or aqueous systems, dissociate to provide melamine resins dissolved or dispersed in the water which contain anionic groups in cocondensed form.
The invention provides water-dilutable etherified melamine-formaldehyde resins obtained by reacting melamine, formaldehyde, a compound having anionic groups and an etherification alcohol where the ratio of the amounts of substance of melamine:formaldehyde:compound having anionic group:etherification alcohol in the reaction mixture is 1:(1.7 to 7.0):(0.01 to 0.4):(10 to 30), said compounds having anionic groups preferably being selected from salts of sulfamic acid (sulfamidic acid, amidosulfuric acid H2Nxe2x80x94SO2xe2x80x94OH), sulfurous acid (H2SO3; HOxe2x80x94SOxe2x80x94OH) and pyrosulfurous acid (disulfurous acid H2S2O5, HOxe2x80x94SOxe2x80x94Oxe2x80x94SOxe2x80x94OH) with alkali metals or ammonium.
Complete or substantially complete reaction provides a resin containing building blocks derived from the compounds mentioned, in the stated mixing ratio. The reaction is termed substantially complete when its degree of conversion is not less than 95%, preferably not less than 97% and especially not less than 98%.
Preference is given to using melamine as a sole amino resin former, but it is also possible to replace up to 10% of the melamine by one or more compounds selected from the group consisting of acetoguanamine, benzoguanamine and caprinoguanamine. Urea and/or thiourea should not be used at all or only in mass fractions of not more than 8%, based on the total mass of the amino resin formers. It is preferable to use not more than 7% and particularly preferably not more than 6% of (thio)urea, most preferred, this mass fraction should not exceed 5%. However, it is possible to use cyclic ureas such as ethyleneurea (2-imidazolidinone), propyleneurea (2-oxohexahydropyrimidine) or glycoluril in mass fractions of up to 10%, based on the total mass of the amino resin formers.
Useful etherification alcohols include linear or branched aliphatic alcohols having 1 to 6, preferably 1 to 4, carbon atoms, used individually or in mixture. Preference is given to using methanol, ethanol, n-propanol, isopropanol, n-butanol and isobutanol; particular preference is given to methanol.
Useful compounds having anionic groups include compounds which, under the conditions of the condensation of melamine and formaldehyde are incorporated into the resultant resins through participation in the condensation reaction or by some other reaction with melamine, formaldehyde or the condensation products of melamine and formaldehyde, and dissociate as the resin dissolves in water to cause anions to be formed in the resin molecules and the cationic constituents of the compounds mentioned to dissociate off into the aqueous solution as a cation. Particularly preferred compounds having anionic groups are the sodium salt of amidosulfuric acid and sodium disulfite (sodium pyrosulfite, sodium metabisulfite).
The invention further provides a process for preparing etherified melamine resins in two stages: first melamine and formaldehyde are reacted under alkaline conditions in the presence of the compound having an anionic group, then the etherification alcohol, preferably methanol, and an acid are added to the resultant resin solution and the etherification is carried on to the desired degree of etherification.
The invention further provides a process wherein the compound having an anionic group is not added until the second stage, after the dissolution of the melamine in the aqueous formaldehyde solution, (i.e., after the onset of the methylolation reaction), or after the conclusion of the methylolation reaction. In either case, however, this compound should still be added during the alkaline condensation phase. The condensation is to be carried on until the level of unconverted compound having anionic group has dropped to less than 2% of the initial concentration. Thereafter, the reaction solution is acidified and the etherification carried out as a third stage.
As criterion for discontinuing the etherification, the compatibility of the reaction mixture with saturated sodium chloride solution can be used throughout all variants. The etherification should preferably be carried on until the compatibility of the reaction mixture with saturated sodium chloride solution is not less than 1:5.0. Compatibility is determined by intimately mixing one part by mass of the reaction mixture with x parts by mass of a saturated sodium chloride solution to provide a homogeneous mixture without visible opacity and increasing the mass of the sodium chloride solution added by gradual addition of this solution before the mixture obtained is again stirred and visually inspected. When there is still no opacity, further sodium chloride solution is added, and so on. A compatibility of 1:x is reported if this is the ratio of the mass of reaction mixture to sodium chloride solution at which permanent opacity first occurs.
The resins thus obtained can be used for preparing building materials, insulation material, fleece, web or nonwovens production, paper and textile finishing and for preparing foils and edgings by impregnation or coating of these substrates with the resin solutions. Mineral materials such as glass wool, rock wool, papers, including decorative papers, cardboard, textile structures such as woven fabric, bonded non-wovens, weft-knitted goods, felts, fleece, each made of fibres or threads comprising glass, natural fibers, fiber-forming natural and synthetic polymers such as cellulose, polyamides, polyesters, polyacrylonitrile or polyolefins being impregnated or coated with aqueous solutions containing these resins. The impregnated or coated materials are subsequently dried and, if appropriate, cut to the desired shape or length. The resins are especially suitable for paper coating and for bonding of glass fibre fleece.
All percentages in the examples hereinbelow as well as supra are mass fractions (ratio of the mass of the material in question to the mass of the mixture), unless otherwise stated. Concentrations in % are mass fractions of the dissolved material in the solution (mass of dissolved material divided by mass of solution).